1. Field of the Invention
The present invention relates to an exhaust gas emission control device for a Diesel engine.
2. Description of the Related Art
A known exhaust gas emission control device for a Diesel engine comprises a honeycomb ceramic filter provided in the exhaust passage to trap particles and to prevent a discharge of particles contained in an exhaust gas to the outside air.
The filter becomes clogged after long term use, and thus must be regenerated. Therefore, a heater means, for example, an electric heater, is provided upstream of the filter, and power is supplied to the heater at a predetermined intervals to burn inflammable particles, for example, carbon particles, trapped in the filter, whereby the filter is regenerated.
In this method of regeneration, however, heat is produced by burning the particles trapped and accumulated on the filter, and if a large amount of particles has accumulated on the filter, the filter can be destroyed, i.e., melted, by the heat of the burning particles.
On the other hand, if only a small amount of particles has accumulated on the filter, the heat is not propagated to the particles at the back of the filter, and thus the regeneration of the filter is not complete.
To solve the above problem, Japanese Unexamined Patent Publication No. 61-223215, disclosed an exhaust gas emission control device which comprises a heater provided near the end surface of the filter, first and second bypass pipes which bypass the filter, and five valves for controlling the flow of exhaust gas in the bypass pipes and the filter. In this device, when particles are to be trapped the exhaust gas does not flow through the first and second bypass pipes, but through the filter and the heater, and is then discharged to the atmosphere. But when the filter is to be regenerated, to ensure that the filter is not melted, the exhaust gas flows through the first bypass pipe, the heater, the filter, and the second bypass pipe, in this order, and is then discharged to the atmosphere, and at the same time, the heater is activated and ignites and burns the trapped particles.
In this device, however, five valves are needed so that the direction of flow of the exhaust gas in the filter can be reversed when regenerating the filter, and these five valves must be controlled, respectively, by five actuators.
In the above-mentioned device, because five valves and five actuators are needed, not only the control of the five actuators but also the construction of this device is complicated, and it is not easy to mount the device to a vehicle.